It is known that knowledge of the intracranial pressure is extremely valuable in many cases in order to monitor and control the clinical condition of a patient (cranial trauma, hydrocephaly, large intracranial lesions . . . ). Presently there are essentially three types of intracranial-pressure sensors:
(a) the sensors requiring handling of the cephalorachitic intraventricular liquid or cisternal liquid, PA1 (b) the so-called sub-dural sensors to be implanted in the subdural space between the dura mater and the arachnoid, PA1 (c) the extra-dural sensors to be implanted on the dura mater between this dura mater and the skull. PA1 a test body provided with a deforming diaphragm supported at its periphery by an essentially undeforming guard ring, PA1 a deformation detector associated with the diaphragm and designed to emit an electric signal which is a function of the deformations of said diaphragm, PA1 and leads attached to the detector to transmit the electrical signal. PA1 the test body composed of diaphragm and guard-ring comprises an integral assembly of a non-magnetic material so that the peripheral part is the guard ring with a thickness suitable to be essentially undeforming and furthermore that the central part is the diaphragm of lesser thickness, PA1 the test body has a continuous front side called the transducing side, extending both to the central part comprising the diaphragm and to its peripheral part comprising the guard ring, with a cavity to the rear of the diaphragm providing the thinning at the side diaphragm, PA1 the elastic mechanical properties of the test-body substance and the dimensional properties of its central part are so matched that in each cross-section of the diaphragm its maximum deflection with respect to the length of the diaphragm in this cross-section be within 0.01 and 0.03 when the diaphragm is subjected to a uniform relative pressure of 30 kilopascals, PA1 the deformation detector includes a strain gauge applied to the inside of the above cavity on that side of the diaphragm which is opposite the transducing side. PA1 the test body is made of a biocompatible plastic transparent to X-rays and with a Poisson coefficient essentially between 0.30 and 0.45 and a Young's modulus essentially between 2.3 and 2.8 gigapascals, PA1 the central part of the test body comprising the diaphragm is essentially between 0.15 and 0.30 mm thick.
The sensors of the first type (a) measure directly the pressure of the cephalorachitic liquid which is transmitted by a catheter to a transducer. The sensors of the second type (b) measure the intracranial pressure by using the arachnoid as an interface, the arachnoid being a very fine and very flexible membrane capable of integrally transmitting the pressures. In both cases the pressure to be measured is directly accessible, without there being distortions, whereby the measurement provides significant information without need for special precautions and while making use of conventional pressure sensors which are merely selected to have the required sensitivity.
However implanting the sensors (a) or (b) entails effraction of the dura mater necessitating a far more complex surgical intervention than that required by an extradural implant (c) and carries risks (parenchymal lesions, meningitis, fistule of the cephalorachitic liquid . . . ) well known to the practitioners; in particular, only the extradural implant is suitable for danger-free, long-term surveillance.
The extradural sensors (c) measure the intracranial pressure indirectly by using the dura mater as the interface; in the light of the advantages of this technique (ease of implantation, possibility of long-term implants, patient-risk suppression . . . ) many sensors of this type have been designed and illustrative examples can be found in the following documents: French Pat. Nos. 2,455,735; 2,384,482; 2,274,261 and 2,262,953; U.S. Pat. Nos. 4,246,908; 4,393,878 and 4,186,749; German Pat. No. 1,965,231.
However control tests have shown that most of the sensors of the type (c) provide spurious outputs without there being an accurate correlation between the measured value and the actual pressure of the cephalorachitic liquid (especially when going from one individual to another). Nevertheless one set of sensors of this type does offer significant measurement values in practice: these are the compensated sensors as described in particular in the French Pat. Nos. 2,455,735 and 2,384,482; the U.S. Pat. No. 4,393,878 and the German Pat. No. 1 965 231. In these latter sensors, a diaphragm is pressed against the dura mater to receive the pressure transmitted by the dura mater on one of its sides, while a compensating system applies a counter-pressure to the opposite side of the diaphragm, this counter-pressure being controlled so as to constantly return the diaphragm to its initial position; the measured parameter is the value of the needed counter-pressure. Such a system results in an exceedingly complex sensor which even when utilizing the most recent technologies cannot be wholly implanted.
In addition, the U.S. Pat. No. 4,186,749 describes a sensor free of the drawbacks attaching to the balanced sensors while still providing meaningful measurements. In this sensor the pressure sensitive diaphragm is metallic with very minute deformations. The main drawback of this sensor is being of low sensitivity and requiring a very sophisticated electronic transducer, whereby its cost is much increased and its bulk is substantial in height, thereby practically making it impossible to place this sensor between the dura mater and the skull. If such a sensor is directly lowered into and placed in the trepan hole, very serious practical difficulties are met in assuring a stable and satisfactory contact with the dura mater; also, it must be anchored on the skull to stay in place.
Accordingly at present there are no extradural implantable sensors providing accurate intracranial pressure information while being simple, whereby they would be economical and offer a bulk compatible with easy and total implantation in the skull between the skull and the dura mater.
The object of the present invention is to remedy this deficiency and to provide a simple sensor which can be easily and extradurally implanted while assuring satisfactory and stable contact with the dura mater, and which furthermore is capable of transmitting an accurate electrical signal representing the intracranial pressure of any individual with an error margin remaining within the bounds generally tolerated by the practitioners (about 5%).
It should be noted that the sensor which is the object of the invention is especially significant when implanted extradurally in the light of the above discussed advantages of this method; nevertheless the geometry of this sensor also is compatible with other implanting modes.